Abstract
Tetrafluoroethylene and butadiene form the 2 + 2 cycloadduct under kinetic control, but the Diels–Alder cycloadduct is formed under thermodynamic control. Borden and Getty showed that the preference for 2 + 2 cycloaddition is due to the necessity for syn-pyramidalization of the two CF2 groups in the 4 + 2 transition state. We have explored the full potential energy surface for the concerted and stepwise reactions of tetrafluoroethylene and butadiene with density functional theory, DFT (B3LYP and M06-2X), DLPNO-UCCSD(T), and CASSCF-NEVPT2 methods and with the distortion/interaction–activation strain model to explain the energetics of different pathways. The 2 + 2 cycloadduct is formed by an anti-transition state followed by two rotations and a final bond formation transition state. Energetics are compared to the reaction of maleic anhydride and ethylene.
Highlights
Cycloadditions are versatile synthetic methods to make cyclic molecules through formation of two carbon−carbon or carbon−heteroatom bonds.[1−4] The theoretical rationalizations and predictions of mechanisms of cycloadditions are significant achievements of Woodward and Hoffmann.[5]
Bartlett and others found that dienes and halogenated ethylenes often give some, or all, 2 + 2 cycloadducts (Scheme 1).[9−11] Figure 1 shows variable temperature studies performed by Weigert and Davis for the reaction of butadiene (2) and tetrafluoroethylene (TFE, 1).[12]
Calculations were performed at the HF/631G* level with an MP2 correction to account for electron correlation
Summary
Cycloadditions are versatile synthetic methods to make cyclic molecules through formation of two carbon−carbon or carbon−heteroatom bonds.[1−4] The theoretical rationalizations and predictions of mechanisms of cycloadditions are significant achievements of Woodward and Hoffmann.[5]. Bartlett and others found that dienes and halogenated ethylenes often give some, or all, 2 + 2 cycloadducts (Scheme 1).[9−11] Figure 1 shows variable temperature studies performed by Weigert and Davis for the reaction of butadiene (2) and tetrafluoroethylene (TFE, 1).[12] Up to 350 °C, only the 2 + 2. (b) Cycloaddition Products of the Reaction of Trifluoroethylene and Butadiene at 215 °C9. The reaction of 1 and 2 was studied theoretically by Borden and Wang 30 years ago. They sought to quantify the π-bond strength of TFE.[13] Calculations were performed at the HF/631G* level with an MP2 correction to account for electron correlation.
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